1. Field of the Invention
The present invention relates to the field of magnetic data read/write operations, and more particularly, to a device to limit current to a read sensor during write operations.
2. Discussion of the Related Art
Disk drives employ giant-magneto-resistive (“GMR”) sensors while performing reading operations on a disk. During read operations, current flows to the GMR sensor from the pre-amplifier. Problems may occur when magnetic instability is experienced within the GMR sensor. Magnetic instability can cause irrecoverable damage to the GMR sensor that degrades performance and durability. Magnetic instability may occur when the current through the sensor exceeds a given threshold for a long duration.
The current in a GMR sensor may be increased inadvertently due to cross-coupling from the writer circuit coupled to the pre-amplifier. This cross-coupling may be due to magnetic fields induced by currents in the circuit loop for the writer signal path that couples inductively onto the circuit loop formed by the reader signal path. This type of cross-coupling is called mutual inductance. Cross-coupling also may be due to electric fields induced by voltage potentials on the writer signal path that couple capacitively onto the reader signal path. This type of cross-coupling is called mutual capacitance. The types of cross-coupling are not mutually exclusive, and both may occur at the same instance.
FIG. 1 depicts read and write signal paths to a recording head. Pre-amplifier 102 supplies current to recording head 104 within a disk drive. A write circuit loop and a read circuit loop couple pre-amplifier 102 and recording head 104. The write circuit loop includes write driver 106, write coil 110 and write signal path 114. The read circuit loop includes read amplifier 108, GMR sensor 112 and read signal path 116. The read signal path and write signal paths that connect the preamp to the head are constructed out of a single piece of flexible printed circuit board that is referred to at the flex 130.
Mutual inductance may occur as follows. A current I2 flows through writer signal path 114. Current I2 induces a magnetic field about write signal path 114. This magnetic field, in turn, induces a voltage VL1 in read signal path 116. Thus, currents I2 in write signal path 114 and voltages VL1 in read signal path 116 are coupled together.
Mutual capacitance may occur as follows. Voltage potentials on writer signal path 114, such as VL2, induce electric fields. The electric fields create a voltage potential, Vc, between the two signal paths and induce a current, IC, to flow into read signal path 116 from write signal path 114. Induced current IC, in turn, becomes current I1 in read signal path 116.
Both mutual inductance and mutual capacitance can induce current I1. Current I1 also flows through the GMR sensor 112 and becomes sensor current Imr. If sensor current Imr already exists, it may be increased by current I1. High current levels can damage GMR sensor 112. Currents greater than 10 mA can cause potential instability in the GMR sensor 112. Further, the magnitude of the current that couples from writer signal path 114 into read signal path 116 will continue to increase as the rise-time of the write signal decreases. The rise-time of the write signal decreases because the rise-time is required to scale inversely with the ever-increasing data rate. The rise-time decrease is desired so that the write current has ample time to induce a field from the thin-film write head onto the magnetic media before the next data transition occurs. Thus, as the data rate increases, the read element will become prone to damage due to coupling from the writer.
One known approach to reduce the coupling effects includes spacing the two sets of signal paths further apart and keeping the individual pairs of signal lines for each signal path closer together. By spacing the read and write signal paths further apart, the capacitance between the two signal paths may be reduced. In addition, the mutual inductance between the two trace loops may be reduced. The mutual inductance may be reduced further by putting the two signal lines of the read signal path 116 closer together. The mutual inductance may also be reduced further by putting the two signal lines of the write signal path 114 closer together. Reducing the area created by each signal path. Shortcomings with this approach include an increased cost associated with the increased area, or width, of the overall read/write circuit as well as the increased cost of using finer geometries for manufacturing the closer-spaced signal pairs of each signal path.
Another approach involves placing a conductive layer on the back side of the flex. This backing layer has the effect of reducing capacitive and magnetic coupling between the two pairs of signals. Shortcomings with this approach include an increased cost related to the more complex manufacturing process of the flex.
Another approach has been adding guard traces between the interconnects. The guard traces act to shield the electric and magnetic fields between the signal paths. Due to the limited area available on the flex these guard traces are ungrounded. A shortfall with adding the ungrounded traces, however, is that charging of these shield lines may impart the additional risk of an electro-static discharge event that could couple into the read lines and result in damage to the GMR sensor.
Thus, attempts to limit the current flowing to the GMR sensor during write operations result in increased costs, increased area or additional risks.